Heiresses were bought and sold

In medieval times, the sovereign and his tenants-in-chief had problems
in raising revenue. They may have had the rent from their own estates but
these were seldom enough to fund their many ambitions and duties. But the
core principle of feudalism was that land was held of the king by the
tenants-in-chief and likewise by lords of manors of a tenant-in-chief. The
feudal superior had the right to withhold the land, though this tended to
be highly unpopular if done in an arbitrary manner. But what was accepted
was the charging of a fee when the land passed from one holder the next.

So heirs to land paid a fee, usually called a fine (=final payment), to
their feudal superior when they inherited the land. Today we would call this
death duties. This was fairly easily handled when the heir was a male
adult, he paid the fine and carried on where his father had. The feudal lord
got his fee quickly

But for minors and heiresses the practice became that the feudal lord took
them into their household and collected the fee when they became adult. This
delayed the receipt of revenue. So they asked others to bid for the
guardianship and surrendered the right to determine the marriage to the new
guardian. This brought the revenue in more quickly.

These guardians bought and sold heirs and heiresses. While their charges
were young, they received the revenues of their estates. If they bought a male
heir they tended to marry them to their daughter, thus providing for their
daughter as the mistress of an appropriate establishment. If they bought a
female heir, an heiress, they could determine who she was to marry and again
might well marry them to their son, thereby enlarging their son's estate.

Obviously there was some friction here and it was not unknown for the
proposed marriages not to occur; the prospective spouses did have the
liberty to refuse to marry, but it would be a brave youngster, of 12 perhaps,
who would exercise this supposed freedom.